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We present the first hybrid-integrated optical phase-lock loop (OPLL) for use in high spectral purity photonic terahertz sources. We have achieved the necessary short loop delay to lock a 1-MHz linewidth slave laser by hybrid integration of the slave laser and photodetector on a silicon motherboard with silica optical waveguides and combining this with a custom-designed low-delay electronic loop filter circuit. The laser and photodetectors are InP-based and are flip chip bonded to silicon daughter boards, which are in turn attached to the motherboard. Delay between the slave laser and photodiode was approximately 50 ps. The heterodyne between slave and master sources has a linewidth of less than 1 kHz and achieved phase noise less than -80 dBc/Hz at an offset of 10 kHz. The slave laser can be offset from the master source by 2-7 GHz, using a microwave oscillator. This integrated OPLL circuit was used with an optical comb source and an injection-locked laser comb filter to generate high spectral purity signals at frequencies up to 300 GHz with linewidths <;1 kHz and powers of about -20 dBm, while the two integrated lasers could deliver a tunable heterodyne signal at frequencies up to 1.8 THz.